Energy demands by the industrialized world are continuing to rise, while the rate of new oil and gas discoveries is falling. Within the next 30 years, available petroleum supplies will fail to meet demand, and oil and gas will no longer be able to serve as the world's major energy source. Other energy sources such as geothermal, solar, and fusion are unlikely to be sufficiently developed to serve as replacements for oil. Coal, on the other hand, exists in relative abundance in the United States, and if it can be adapted to use in existing applications which have been engineered for petroleum use, it can serve as an inexpensive substitute for, and successor to, the more expensive petroleum fuels in use today. In order to be used as a petroleum substitute, however, the coal must be converted to a fluid state, so that systems burning gas, fuel oil, diesel fuel, and other petroleum products can be adapted to its use with minimal equipment modification. The coal also must be cleaned, or purged of its mineral matter (ash precursor) content, including its sulfur (pyrite) content, to eliminate or minimize corrosion, erosion, slagging or fouling of equipment, to minimize the need for post-combustion gas clean-up to meet environmental standards, and to increase fuel value per pound.
U.S. Pat. No. 4,169,710 discloses that treating raw, lump coal with high concentrations of hydrogen fluoride in liquid or gaseous form removes much of the ash content, and this removal of ash from the interstices within the coal tends to cause the coal to break up, so that the hydrogen fluoride also serves as a comminuting agent to produce coal fines.
A major difficulty with previous hydrogen fluoride leach processes has been the relative insolubility of certain fluorides of the alkali metals and alkaline earths (for example, CaF.sub.2 and MgF.sub.2). Since the cations Ca.sup.2+, Mg.sup.2+, Na.sup.+ and K.sup.+ are abundant in the mineral matter typically accompanying coal, the consequence of the HF leaching of coal has been the formation of insoluble fluorides. In the product from such a leach, these insoluble compounds comprise ash precursors. Therefore, the efficiency of the ash reduction process is diminished by formation of insoluble fluoride compounds. Furthermore, the insoluble fluorides which exit the process with the beneficiated coal comprise a loss of fluoride from the system which must be compensated with alternate fluoride materials at additional expense. Moreover, when beneficiated coal containing fluorides is fired, it creates corrosion problems in the firing equipment. Additionally, the fluoride in the combustion gases constitutes a potential environmental threat. Finally, when the HF-beneficiated coal product containing alkali metal and alkaline earth fluorides is used as a fuel in heat engines, the cations of these insoluble fluorides, especially sodium and potassium cations, are quite damaging to the internal parts of heat engines, in particular gas turbines.
This situation presented an apparent dilemma to those practicing the prior art. On one hand, dissolving the aluminosilicate minerals commonly associated with coal was thought to require a high concentration of hydrofluoric acid (and, consequently, free fluoride ions). On the other hand, a concentration of free fluoride ions results in formation of highly insoluble alkaline earth fluorides and/or alkali metal fluorides.
One method which has been used in an attempt to solve the problem of the production of insoluble fluorides, has been to employ a different acid, often HCl alone, in a pre-leach, and/or in a subsequent leach. Such an HCl leach is effective in dissolving only some of these insoluble fluorides. For example, Na.sub.3 AlF.sub.6 is substantially insoluble in HCl. Further, such an HCl leach is effective only when it is used alone, i.e. unmixed with HF. A difficulty with these answers to the insoluble fluoride problem is that the acid regeneration cycle becomes more complicated. In particular, it is necessary to provide two or more separate regeneration cycles for the different acids and acid mixtures. Accordingly, it would be desirable to provide an efficient method for leaching coal feed which contains insoluble fluoride-producing cations, such as Na, K, Ca, and/or Mg, using only HF or a mixture of HF and another acid such as HCl, but which both eliminates the need for a separate HCl pre-leach and regeneration cycle for HCl and avoids the formation of insoluble fluorides.
Because of the relative insolubility of many alkali metal and alkaline earth fluorides, prior methods of leaching with HF or an HF/HCl mixture have been less effective in removing basic ash minerals CaO, MgO, Na.sub.2 O, K.sub.2 O and Fe.sub.2 O.sub.3 (expressed as the ash oxides) than acidic ash minerals SiO.sub.2, Al.sub.2 O.sub.3 and TiO.sub.2 (expressed as the ash oxides). Such difference in effectiveness of leaching causes an increase in the ratio of basic ash mineral to acidic ash mineral in the leach product of a conventional leach process. This is significant because the tendency of ash to slag or foul is increased as the ratio of basic ash oxides to acidic ash oxides increases. Thus, it would be desirable to provide a method for leaching a coal feed so as to produce a product with a low ratio of basic ash oxides to acidic ash oxides. To do so requires efficient leaching of basic minerals.
Accordingly, it is an object of this invention to provide a means for producing a reduced-ash coal product by leaching with a fluoride-containing acid while minimizing loss of fluorine values to insoluble alkaline earth fluorides and/or while eliminating the need for a separate HCl pre- or post-leach.
It is a further object of this invention to provide a leaching process which dissolves substantially all alkaline earth minerals, particularly calcium, occurring in the leach feed.
It is still a further object of this invention to produce a coal product using a leaching process which dissolves substantially all the alkali metal-containing minerals, particularly the sodium- and potassium-containing minerals, occurring in the leach feed.
It is also an object of this invention to provide a leaching process which produces a coal product with a lower ratio of basic ash oxides to acidic ash oxides than obtained by conventional HF leaching.
It is another object of this invention to provide a process for producing a reduced-ash coal product which includes an acid regeneration circuit, but wherein the amount of fluorine lost as alkali metal or alkaline earth fluoride is substantially reduced, and insoluble fluorides normally admixed in the coal product are substantially reduced or virtually eliminated.
It is yet another object of this invention to provide an improved process for producing a reduced-ash coal product wherein the improvement comprises adjusting concentrations of Al or Si species relative to the concentration of free-fluoride-ions so as to substantially prevent or reverse loss of fluorine values as insoluble fluorides, and so as to substantially eliminate insoluble fluorides in the final coal product.
It is still another object of this invention to provide a process for producing a reduced alkaline earth and/or reduced alkali metal coal product.
It is also an object to produce a finely-ground purged coal product usable not only as a substitute for petroleum fuels, e.g., as a boiler, diesel or turbine fuel, but also as a substitute for activated carbon, or as a feedstock for activated carbon, carbon black, electrode carbon, and various chemical processes.